JP2010218834A - Nonaqueous electrolyte secondary battery and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an anode material with a high capacity density per volume important for enhancement of capacity of a nonaqueous electrolyte secondary battery, through development of a material not only with a higher capacity density per weight but with a higher true density than graphite, in developing a material with a larger true specific gravity than the graphite in order to provide a high-capacity anode material indispensable for enhancement of capacity of a lithium-ion battery. <P>SOLUTION: For the nonaqueous electrolyte secondary battery provided with a work electrode 1 equipped with an anode active material, a counter electrode 2, and nonaqueous electrolyte, the anode active material contains Na<SB>x</SB>FeO<SB>2</SB>(0.8≤x≤1.2) having a lamellar structure. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium ion battery or a polymer battery, and more particularly to a non-aqueous electrolyte secondary battery capable of achieving high capacity and a method for manufacturing the same.

  In recent years, mobile information terminals such as mobile phones, notebook personal computers, and PDAs have been rapidly reduced in size and weight, and batteries as drive power sources are required to have higher capacities. A non-aqueous electrolyte secondary battery that performs charge / discharge by moving lithium ions between the positive and negative electrodes along with charge / discharge has a high energy density and a high capacity. Widely used as a drive power source.

  Here, the mobile information terminal has a tendency to further increase power consumption with enhancement of functions such as a video playback function and a game function, and the non-aqueous electrolyte secondary battery, which is a driving power source thereof, plays back and outputs for a long time. For the purpose of improvement and the like, further increase in capacity and performance are strongly desired. In addition, non-aqueous electrolyte secondary batteries are expected to be used not only for the above applications, but also for power tools, assist bicycles, and HEVs. Capacity and high performance are strongly desired.

  Here, in order to increase the capacity of the nonaqueous electrolyte secondary battery, it is essential to increase the capacity of the negative electrode, and graphite has been proposed as a negative electrode material (see Patent Document 1 below).

Japanese Patent Laid-Open No. 05-028996

  However, the capacity of graphite is 344 mAh / g, and in order to increase the capacity of the nonaqueous electrolyte secondary battery, development of a negative electrode material having a larger capacity is required.

  Therefore, the present invention develops a material having a true specific gravity larger than that of graphite in order to provide a high-capacity negative electrode material indispensable for increasing the capacity of a lithium ion battery. In addition to the capacity density per weight, An object of the present invention is to provide a negative electrode material having a high capacity density per volume, which is important for increasing the capacity of a non-aqueous electrolyte secondary battery, by developing a material having a density higher than that of graphite.

In a nonaqueous electrolyte secondary battery including a negative electrode having a negative electrode active material, a positive electrode, and a nonaqueous electrolyte, the negative electrode active material includes Na _x FeO ₂ having a layered structure (0.8 ≦ x ≦ 1.2). Is included.
When Na _x FeO ₂ having a layered structure is used as the negative electrode active material as in the above configuration, lithium ions are reversibly inserted and desorbed between the layers without extracting sodium of NaFeO ₂ , Trivalent and zero-valent changes can be used. Thereby, it is considered that a large charge / discharge capacity can be obtained.
In addition, since sodium in Na _x FeO ₂ does not participate in charging / discharging, sodium does not desorb from the negative electrode active material during charging / discharging. Therefore, even when the sodium-containing compound is used as the negative electrode active material of the non-aqueous electrolyte secondary battery, no problem occurs during charging / discharging.

Here, in comparison with the case where graphite is used as the negative electrode active material, the effect of the present invention will be described. Na _x FeO ₂ has a higher discharge capacity density of the negative electrode active material per unit mass than graphite. Moreover, the true density is also increasing. Therefore, Na _x FeO ₂ has a higher discharge capacity density of the negative electrode active material per unit volume obtained by the following equation (1) than graphite. As a result, the battery discharge capacity can be increased while the battery is downsized.

Discharge capacity density of negative electrode active material per unit volume = Discharge capacity density of negative electrode active material per unit mass × true density (1)

In general, NaFeO ₂ (sodium ferrite) is known as a layered compound, and proposals have been made to use NaFeO ₂ as an active material. Conventional proposals use NaFeO ₂ as a positive electrode active material. Therefore, it is added that it is not used as a negative electrode active material as in the present invention.

For example, JP-A-8-124600 and J.H. AM. CHEM. SOC. As described in 2008, V130, 3554, etc., use of NaFeO ₂ sodium as a positive electrode active material by ion exchange with sodium is under study. However, in these proposals, lithium is inserted and desorbed using the change between trivalent and tetravalent iron, and the charge / discharge capacity is as small as about 130 mAh / g. Further, as disclosed in JP-A-2001-283852, it has been studied to use a mixture of vanadium oxide and NaFeO ₂ as a positive electrode active material. In this specification, NaFeO ₂ is used alone as a positive electrode. It has been reported that the electrode used as the active material hardly charges or discharges.
As described above, when NaFeO ₂ (including this sodium ion-exchanged lithium) is used as the positive electrode active material, the charge / discharge capacity of the nonaqueous electrolyte secondary battery cannot be significantly improved.

The negative electrode active material is preferably composed only of Na _x FeO ₂ (0.8 ≦ x ≦ 1.2).
As the negative electrode active material, graphite or the like may be contained in addition to Na _x FeO _2. However, in order to maximize the effects of the present invention, the above configuration is desirable.

It is desirable that the value of x is 1.
Moreover, it is desirable that at least a part of the Na _x FeO ₂ (0.8 ≦ x ≦ 1.2) has a β-NaFeO ₂ type structure.
Both α-NaFeO ₂ and β-NaFeO ₂ have a layered structure, but the former space group is R-3m, whereas the latter space group is Pna21. When the negative electrode active material is only α-NaFeO ₂ , lithium insertion / extraction is not performed with good reversibility, and the discharge capacity density may be reduced. Therefore, it is preferable that the negative electrode active material contains a β-NaFeO ₂ type structure.

Na _x FeO ₂ (0.8 ≦ x ≦ 1.2) is prepared by mixing a sodium source and an iron source to prepare a mixture, and firing the mixture in a temperature range of 500 ° C. to 900 ° C. A negative electrode active material containing Na _x FeO ₂ (0.8 ≦ x ≦ 1.2), a conductive agent, and a binder, and the negative electrode and positive electrode And a step of disposing the power generation element disposed through the separator together with the non-aqueous electrolyte in the battery exterior body.
The nonaqueous electrolyte secondary battery described above can be manufactured by such a manufacturing method.

[Other matters]
(Matters related to negative electrode)
(1) The sodium source used in the preparation of the negative electrode active material is at least one selected from the group consisting of sodium carbonate, sodium acetate, sodium hydroxide, sodium nitrate, sodium oxide, sodium peroxide, sodium oxalate and the like. Further, the iron source is selected from the group consisting of the above-mentioned ferric trioxide (Fe ₂ O ₃ ), triiron tetroxide (Fe ₃ O ₄ ), iron oxyhydroxide (FeOOH), and the like. At least one may be used. Furthermore, it is preferable that both baking temperature is 500 to 900 degreeC.

(2) As the conductive agent, carbonaceous materials, metals, semiconductors, metal carbides, metal compounds, and the like can be used. Among these, if a material that absorbs and releases lithium is used as the conductive agent, the capacity density of the negative electrode is increased. Since it increases, it is especially preferable to use the electrically conductive agent which consists of the said material. Examples of the carbonaceous material include artificial graphite, natural graphite, acetylene black, and carbon black. Examples of the metal include tin, gallium, and aluminum. Further, silicon is an example of the semiconductor, and examples of the metal carbide are those having electrical conductivity such as titanium carbide, tantalum carbide, tungsten carbide, zirconium carbide.
(3) As the binder, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene butylene rubber (SBR), polyacrylonitrile (PAN), or the like can be used.

(Matters related to positive electrode)
(1) It is preferable to use a material capable of occluding and releasing lithium as the positive electrode active material. For example, a lithium-containing transition metal oxide or a lithium-containing transition metal phosphor generally known as a positive electrode of a lithium ion battery is used. It is desirable to use an acid compound or the like. Examples of the lithium-containing transition metal oxide include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _{4 and the} like, and examples of the lithium-containing transition metal phosphate compound include LiFePO ₄ , LiMnPO ₄ , LiCoPO _{4 and the} like. Can be mentioned.

(Matters related to non-aqueous electrolytes)
(1) Examples of the nonaqueous electrolyte solvent used in the present invention include cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles, amides, and the like. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate and the like, and those in which some or all of these hydrogens are fluorinated can also be used. Examples thereof include trifluoropropylene carbonate and fluoroethylene carbonate. Examples of the chain carbonic acid ester include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, and the like. It is possible to use. Examples of the esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone. Examples of the cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,4-dioxane, 1,3, 5-Trioxane, furan, 2-methylfuran, 1,8-cineol, crown ether and the like can be mentioned. Examples of the chain ethers include 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, and pentyl. Phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1, 1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetrae Examples include tylene glycol dimethyl. Examples of the nitriles include acetonitrile, and examples of the amides include dimethylformamide. And at least 1 sort (s) selected from these can be used.

(2) As the lithium salt added to the non-aqueous solvent, those generally used as an electrolyte in a conventional non-aqueous electrolyte secondary battery can be used. For example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (FSO ₂ ) ₂ , LiN (C _l F _{2l + 1} SO ₂ ) (C _m F _{2m + 1} SO ₂ ) (l, m is an integer of 1 or more), LiC (C _p F _{2p + 1} SO ₂ ) (C _q F _{2q + 1} SO ₂ ) (C _r F _{2r + 1} SO ₂ ) (p, q, r are integers of 1 or more), Li [B (C ₂ O ₄ ) ₂ ] (bis (oxalate) lithium borate ( LiBOB)), Li [B (C ₂ O ₄ ) F ₂ ], Li [P (C ₂ O ₄ ) F ₄ ], Li [P (C ₂ O ₄ ) ₂ F ₂ ] and the like. Use one type of lithium salt You may use, and you may use it in combination of 2 or more types.

(Matters related to other battery components)
(1) The non-aqueous electrolyte battery according to the present invention has a battery configuration such as a positive electrode active material, a negative electrode active material, a non-aqueous electrolyte, a separator, a battery case, and a current collector that holds the active material and carries out current collection. Consists of members. The constituent elements other than the positive electrode active material and the electrolyte are not particularly limited, and various known members may be selectively used.

  According to the present invention, not only the capacity density per weight but also the material whose true density is higher than that of graphite is used to improve the capacity density per volume in the negative electrode. There is an excellent effect that the capacity can be increased.

It is sectional drawing of the test cell which concerns on the form for implementing this invention. It is a graph which shows the XRD measurement result of the negative electrode active material used for this invention cell A1. It is a graph which shows the XRD measurement result of the negative electrode active material used for this invention cell A2. It is a graph which shows the XRD measurement result of the negative electrode active material used for this invention cell A3. It is a graph which shows the XRD measurement result of the negative electrode active material used for this invention cell A4. It is a graph which shows the XRD measurement result of the negative electrode active material used for this invention cell A5. It is a graph which shows the XRD measurement result of the negative electrode active material used for the comparison cell X2. It is a graph which shows the XRD measurement result before and behind ion exchange of the negative electrode active material used for the comparison cell X3. It is a graph which shows the charging / discharging characteristic in this invention cell A1. It is a graph which shows the charging / discharging characteristic in this invention cell A2. It is a graph which shows the charging / discharging characteristic in this invention cell A3. It is a graph which shows the charging / discharging characteristic in this invention cell A4. It is a graph which shows the charging / discharging characteristic in this invention cell A5. It is a graph which shows the charging / discharging characteristic in the comparison cell X1. It is a graph which shows the charging / discharging characteristic in the comparison cell X2. It is a graph which shows the charging / discharging characteristic in the comparison cell X3.

  Hereinafter, the nonaqueous electrolyte secondary battery according to the present invention will be described with reference to FIG. In addition, the nonaqueous electrolyte secondary battery in this invention is not limited to what was shown to the following form, In the range which does not change the summary, it can change suitably and can implement.

(Production of working electrode)
First, Na ₂ CO ₃ and Fe ₂ O ₃ mixed at a molar ratio of 1: 1 and pelletized are fired at 900 ° C. for 12 hours in the atmosphere to synthesize NaFeO ₂ as a negative electrode active material. did. Next, 70% by mass of NaFeO ₂ , 25% by mass of artificial graphite as a conductive agent, and 5% by mass of polyvinylidene fluoride (PVdF) as a binder are mixed to prepare a negative electrode active material slurry. Then, this negative electrode active material slurry was applied onto a copper foil having a thickness of 10 μm by a doctor blade method to produce an electrode plate. Next, this electrode plate was cut into a size of 2 cm × 2 cm, and further vacuum-dried at 105 ° C. for 2 hours to produce a working electrode (negative electrode) 1.

[Production of counter electrode and reference electrode]
A counter electrode (positive electrode) 2 and a reference electrode 4 were produced by cutting out a lithium metal plate into a predetermined size and attaching a tab thereto.
(Preparation of non-aqueous electrolyte)
Non-water is obtained by dissolving lithium hexafluorophosphate (LiPF ₆ ) at a ratio of 1 mol / liter in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of 3: 7. An electrolyte was prepared.

[Production of test cell]
After placing the counter electrode 2, the separator 3, the working electrode 1, the separator 3, and the reference electrode 4 in a test cell container 5 made of a laminate film in an inert atmosphere, the nonaqueous electrolyte is placed in the test cell container 5. The test cell shown in FIG. 1 was produced by pouring the liquid. A part of the lead 6 protrudes from the test cell container 5.

Example 1
A test cell was produced in the same manner as in the embodiment for carrying out the invention.
The test cell thus produced is hereinafter referred to as the present invention cell A1.
Here, since the XRD measurement of the negative electrode active material used in the cell A1 of the present invention (the source is CuKα, the measurement range 2θ = 10 ° to 80 °) is performed, the XRD pattern is shown in FIG. As is clear from FIG. 2, the negative electrode active material used in the cell A1 of the present invention has a peak at the same angle as β-NaFeO _2, and thus the negative electrode active material was found to be β-NaFeO ₂ .

(Example 2)
A test cell was produced in the same manner as in Example 1 except that the firing temperature for producing the negative electrode active material was set to 700 ° C.
The test cell thus produced is hereinafter referred to as the present invention cell A2.

Here, since the XRD measurement (the radiation source is CuKα, the measurement range 2θ = 10 ° to 80 °) of the negative electrode active material used in the cell A2 of the present invention was performed, the XRD pattern is shown in FIG. As is clear from FIG. 3, since the negative electrode active material used in the cell A2 of the present invention includes both α-NaFeO ₂ and β-NaFeO ₂ peaks, it was found that these were mixed NaFeO ₂ . . Further, when comparing the peak intensity ratio [α-NaFeO ₂ main peak (peak at about 18 °) and β-NaFeO ₂ main peak (about 34 ° peak)], the β-NaFeO ₂ main peak is more Since it is high, it can be considered that the proportion of β-NaFeO ₂ is higher than that of α-NaFeO ₂ in the negative electrode active material of the present invention cell A2.

Example 3
Na ₂ CO ₃ and Fe ₃ O ₄ mixed at a molar ratio of 3: 2 into pellets were baked in the atmosphere at 700 ° C. for 12 hours to synthesize NaFeO ₂ as the negative electrode active material. Produced a test cell in the same manner as in Example 1 above.
The test cell thus prepared is hereinafter referred to as the present invention cell A3.

Here, since the XRD measurement of the negative electrode active material used in the cell A3 of the present invention (the source is CuKα, the measurement range 2θ = 10 ° to 80 °) is performed, the XRD pattern is shown in FIG. As is clear from FIG. 4, since the negative electrode active material used in the cell A3 of the present invention includes both α-NaFeO ₂ and β-NaFeO ₂ peaks, it was found that these were mixed NaFeO ₂ . . Further, when the peak intensity ratio (same as in Example 2 above) is compared, the main peak of α-NaFeO ₂ is relatively higher than that of the negative electrode active material of the cell A2 of the present invention. In the negative electrode active material, it is considered that the proportion of α-NaFeO ₂ contained is larger than that of the negative electrode active material of the cell A2 of the present invention.

Example 4
A mixture of NaOH and FeOOH in a molar ratio of 1: 1 and pelletized is baked in air at 350 ° C. for 20 hours, and then kneaded again into pellets at 500 ° C. for 20 hours in air. A test cell was prepared in the same manner as in Example 1 except that NaFeO ₂ as the negative electrode active material was synthesized by firing.
The test cell thus prepared is hereinafter referred to as the present invention cell A4.

Here, since XRD measurement (the radiation source is CuKα, measurement range 2θ = 10 ° to 80 °) of the negative electrode active material used in the cell A3 of the present invention was performed, the XRD pattern is shown in FIG. As is clear from FIG. 5, since the negative electrode active material used in the cell A4 of the present invention includes both α-NaFeO ₂ and β-NaFeO ₂ peaks, it was found that these were mixed NaFeO ₂ . . Further, when comparing the peak intensity ratio (similar to Example 2 above), the ratio of α-NaFeO ₂ and β-NaFeO ₂ is considered to be between the present invention cell A2 and the present invention cell A3.

(Example 5)
A test cell was fabricated in the same manner as in Example 4 except that FeOOH was changed to Fe ₃ O ₄ .
The test cell thus produced is hereinafter referred to as the present invention cell A5.
Here, since the XRD measurement (the radiation source is CuKα, the measurement range 2θ = 10 ° to 80 °) of the negative electrode active material used in the cell A5 of the present invention was performed, the XRD pattern is shown in FIG. As apparent from FIG. 6, since it has a peak in the negative electrode active material same angle as alpha-NaFeO ₂ used in the present invention the cell A5, the negative active material was found to be alpha-NaFeO _2.

(Comparative Example 1)
Comparative Example 1 is significantly different from Example 1 in that NaFeO ₂ synthesized in the same manner as in Example 1 was used as the positive electrode active material. A specific cell was manufactured as follows.
(Production of working electrode)
First, a positive electrode active material slurry was prepared by mixing 70% by mass of synthesized NaFeO ₂ , 25% by mass of artificial graphite as a conductive agent, and 5% by mass of PVdF as a binder. The electrode plate was produced by applying by a doctor blade method on a 15 μm aluminum foil. Next, this electrode plate was cut into a size of 2 cm × 2 cm, and further vacuum-dried at 105 ° C. for 2 hours to produce a working electrode (positive electrode).

[Production of counter electrode and reference electrode]
A counter electrode (negative electrode) and a reference electrode were produced by cutting out a lithium metal plate to a predetermined size and attaching a tab thereto.
(Preparation of non-aqueous electrolyte)
A nonaqueous electrolyte was prepared in the same manner as in Example 1.

[Production of test cell]
A test cell was prepared in the same manner as in Example 1 above.
The test cell thus fabricated is hereinafter referred to as a comparison cell X1.

(Comparative Example 2)
A mixture of Li ₂ CO ₃ and Fe ₂ O _{3 in} a molar ratio of 1: 1 and pelletized was baked in the atmosphere at 900 ° C. for 12 hours to synthesize LiFeO ₂ as a negative electrode active material. A test cell was prepared in the same manner as in Example 1.
The test cell thus produced is hereinafter referred to as a comparison cell X2.
Here, since the XRD measurement (the source is CuKα, the measurement range 2θ = 10 ° to 80 °) of the negative electrode active material used in the comparative cell X2, the XRD pattern is shown in FIG. As apparent from FIG. 7, the negative electrode active material used in Comparative cell X2 is because it has a peak at the same angle and LiFeO _2, the negative electrode active material was found to be LiFeO _2.

(Comparative Example 3)
A test cell was prepared in the same manner as in Example 1 except that LiFeO ₂ having the same crystal structure as Example 1 was synthesized and used as the negative electrode active material. A specific negative electrode active material was synthesized by first synthesizing NaFeO ₂ by the method of Example 1, then adding 50 g of the mixed NaFeO ₂ in a ratio of LiNO ₃ : LiCl = 88: 12 to 50 g. Then, by holding at 280 ° C. for 10 hours, sodium is ion-exchanged with lithium to synthesize LiFeO ₂ .
The test cell thus produced is hereinafter referred to as a comparison cell X3.

  Here, since the XRD measurement of the negative electrode active material used in the comparative cell X2 (the source is CuKα, the measurement range 2θ = 10 ° to 80 °) is performed, the XRD pattern is shown in FIG. As is clear from FIG. AM. CHEM. SOC. As reported by Bruce et al., 2008, V130, 3554, it can be confirmed that each peak is shifted, and it is considered that sodium was ion-exchanged with lithium while maintaining the crystal structure.

(Comparative Example 4)
A test cell was prepared in the same manner as in Example 1 except that graphite was used as the negative electrode active material.
The test cell produced in this way is hereinafter referred to as a comparison cell X4.

(Experiment)
Since the charge / discharge characteristics (charge / discharge capacity density) of the cells A1 to A5 and the comparative cells X1 to X3 were examined, the results are shown in FIGS. 9 to 16 and Table 1, respectively (FIG. 9 shows the present invention). 10 is the present invention cell A3, FIG. 12 is the present invention cell A4, FIG. 13 is the present invention cell A5, FIG. 14 is the comparison cell X1, FIG. 15 is the comparison cell X2, and FIG. 16 is a graph of the comparison cell X3). The charge / discharge capacity density was obtained by dividing the charge / discharge capacity of the cell by the weight of the mixture (active material + conductive agent + binder). Further, the discharge capacity density and the true density per gram of the active material excluding graphite were examined, and the discharge capacity density per 1 cm ^{3 of} the active material excluding graphite was calculated from these values using the above equation (1). Therefore, those results are also shown in Table 1.

Here, the charge and discharge conditions of the cells except the comparative cell X1 is, 0.125 mA ^/ cm 2 were charged at a current of (0.05 It equivalent) until the charging potential ^{0V (vsLi / Li +),} 0.125mA / cm 2 This is a condition that the battery is discharged to a discharge potential of 2.0 V (vsLi / Li ⁺ ) with a current of 0.05 It (equivalent to 0.05 It). The charge / discharge condition of the comparative cell X1 is 0.125 mA / cm ² (0 after charging to a charge potential of 4.5 V (vsLi / Li ⁺ ) with a current of 0.125 mA / cm ² (equivalent to 0.1 It). .1 It) is a condition of discharging to a discharge potential of 2.0 V (vsLi / Li ⁺ ). The only difference between the charge and discharge conditions of the comparison cell X1 is that the charge / discharge capacity density of the negative electrode active material is examined in cells other than the comparison cell X1, whereas the charge / discharge capacity density of the positive electrode active material is examined in the comparison cell X1. Because.

As apparent from FIGS. 9 to 13 and Table 1, in the cells A1 to A5 of the present invention, the charge capacity density is 768 mAh / g to 896 mAh / g, the discharge capacity density is 348 mAh / g to 553 mAh / g, and the negative electrode active material A discharge capacity density higher than that of the comparative cell X4 (discharge capacity density of 344 mAh / g) made of graphite was obtained. On the other hand, as is clear from FIGS. 14 to 16 and Table 1, in the comparative cells X1 to X3, the charge capacity density is 10 mAh / g to 761 mAh / g, the discharge capacity density is 8 mAh / g to 186 mAh / g, It can be seen that the material has a lower discharge capacity density than the comparative cell X4 made of graphite. From the above, it can be seen that the negative electrode active material preferably contains Na _x FeO ₂ having a layered structure.

Here, the reason why the charge / discharge capacity was slightly obtained in the comparative cell X1 is considered to be that graphite used as a conductive agent at the time of charging was doped with anions, and 2V to 4.5V (vs. It is considered that sodium is not desorbed from NaFeO ₂ in the potential range of Li / Li ⁺ ). Therefore, in considering the charge and discharge characteristics in the present invention material A1 to A5, lithium is inserted into NaFeO ₂ is charging reaction to ^{0V (vs.Li/Li +), 2V (} vs.Li/Li +) In the discharge reaction up to this point, it is appropriate to understand that only lithium inserted by charge is desorbed and sodium does not act as an active material in the battery system of the present invention.

In addition, the discharge capacity density of the comparative cell X2 is significantly lower than those of the cells A1 to A5 of the present invention because LiFeO ₂ as the negative electrode active material is not a layered compound, so that lithium is reversibly inserted and released. This is thought to be because there was not.
Furthermore, in the comparative cell X3, the negative electrode active material has a layered structure. However, since sodium is ion-exchanged with lithium, the discharge capacity density may be significantly reduced as compared with the cells A1 to A5 of the present invention. all right. From this, sodium is not directly involved in charge / discharge, but is considered to be an important element for obtaining a large discharge capacity density.

In addition, it is recognized that the discharge capacity density of the present invention cell A5 is lower than that of the present invention cells A1 to A4. This is considered to be due to the following reasons. The negative electrode active materials α-NaFeO ₂ and β-NaFeO ₂ both have a layered structure, but the space groups are R-3m and Pna21, which are different from each other. Therefore, due to the difference in crystal structure between the two, in the cell A5 of the present invention using only α-NaFeO ₂ as the negative electrode active material, lithium insertion / extraction is not reversibly performed and the discharge capacity density is lowered. It is thought that. However, the present invention cell A2~A4 is be a mixture and the beta-NaFeO ₂ and alpha-NaFeO _2, mixing ratio of the alpha-NaFeO ₂ and beta-NaFeO ₂ are different. Thus, although the mixing ratios of the two are different, no significant difference is observed in the discharge capacity density in the cells A2 to A4 of the present invention. Therefore, if β-NaFeO ₂ is contained even slightly, α- It is considered that insertion / extraction of lithium to / from NaFeO ₂ is performed with good reversibility.

Further, as apparent from Table 1, in the present invention cells A1 to A5, the discharge capacity density per gram of the active material excluding graphite was 369 mAh / g, the true density was 3.83 g / cm ³ , and graphite (active It can be seen that the discharge capacity density per gram of the substance is higher than 362 mAh / g, and the true density is higher than 2.25 g / cm ³ ). From this, in the present invention cells A1 to A5, the discharge capacity density per cm ^{3 of} active material excluding graphite is 1412 to 2533 mAh / cm ³ , and the discharge capacity density per cm ^{3 of} active material is 814 mAh / cm 3. ³ ) It is higher.

On the other hand, in comparison cells X1 and X2, the true density is 4.14 g / cm ^3, which is higher than graphite, but the discharge capacity density per 1 g of active material excluding graphite is 33 to 137 mAh / g. It can be seen that it is significantly lower than graphite. From this, in the comparison cells X1 and X2, the discharge capacity density per 1 cm ^{3 of} the active material excluding graphite is 136 to 567 mAh / cm ^3, which is lower than that of graphite.

  The present invention can be applied to, for example, a driving power source of a mobile information terminal such as a mobile phone, a notebook computer, and a PDA.

1: Working electrode 2: Counter electrode 3: Separator 4: Reference electrode 5: Test cell 6: Lead

Claims (5)

In a non-aqueous electrolyte secondary battery comprising a negative electrode having a negative electrode active material, a positive electrode, and a non-aqueous electrolyte,
The non-aqueous electrolyte secondary battery, wherein the negative electrode active material contains Na _x FeO ₂ (0.8 ≦ x ≦ 1.2) having a layered structure. The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material is made of only Na _x FeO ₂ (0.8 ≦ x ≦ 1.2).   The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the value of x is 1. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein at least a part of the Na _x FeO ₂ (0.8 ≦ x ≦ 1.2) has a β-NaFeO ₂ type structure. 5. . Mixing a sodium source and an iron source to make a mixture;
Baking Na _x FeO ₂ (0.8 ≦ x ≦ 1.2) by firing the mixture in a temperature range of 500 ° C. to 900 ° C .;
Producing a negative electrode using a negative electrode active material containing Na _x FeO ₂ (0.8 ≦ x ≦ 1.2), a conductive agent, and a binder;
A step of disposing a power generation element in which the negative electrode and the positive electrode are disposed via a separator together with a non-aqueous electrolyte in a battery exterior body;
A method for producing a nonaqueous electrolyte secondary battery, comprising:

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